This invention relates to rotary drill bits for use in drilling and coring deep holes in subsurface formations. More particularly, the invention pertains to apparatus and methods for mounting stud cutters on the bodies of drag bits, and may have application to cutter inserts mounted to rock bit cones, as well as to the mounting of fluid nozzles to the bodies of both types of bits.
A rotary drill bit, of the kind to which the invention relates, comprises a bit body having a shank for connection of the bit to a drill string. Typically, the bit body contains an inner passageway for introducing drilling fluid to the face of the bit. The bit body is typically formed of steel or of a metal matrix including hard, wear-resistant particles such as tungsten carbide infiltrated with a hardenable liquid binder. Mounted in receptacles within a drag bit body is a plurality of insert stud cutters and/or slug cutters, together with nozzles for introducing drilling fluid to the cutters for cooling, lubrication and removing particles of drilled material. Similarly, cutter inserts are secured within apertures in the exteriors of the rotating cones of rock bits.
When compared with the earlier-developed conventional mill tooth rock bits, cutter inserts of tungsten carbide or diamond may have a tendency to become dislodged from their insert holes in a roller cone. Similarly, slug cutters and stud cutters may have a tendency to separate from a drag bit body. One reason for this is that the bit body or cone body cannot be hardened to the same high Rockwell hardness level as conventional mill tooth bits, because of the lower hardness required for drilling the cutter sockets or insert holes. As a result of the lower hardness of the bit body or cone body at the surface and particularly the subsurface portions thereof, erosion from the circulating mud may occur more rapidly, and eventually the cutter or insert may come loose. Thus, cutters or inserts which are conventionally brazed into sockets insert holes have a relatively high frequency of loss. The cutters and inserts fall out, leaving a clean hole in the bit or cone and eventually leading to bit failure as the uncut segment of the formation previously contacted by the now-missing cutter or insert disrupts the design cutting action of the bit.
Breakage of cutters is another common problem in rock drilling and necessitates removal and replacement of the defective cutter stud, cutter slug or insert from its socket. Such replacement is not always readily accomplished in the field with prior art insert affixation techniques, where the required specialized tools are often unavailable.
Finally, replaceable nozzles have been commercially available for many years, but state-of-the art nozzle affixation structures leave much to be desired in terms of ease of removal and placement of nozzles.